1. Feline Panleukopenia Virus Biology and Pathology
Feline panleukopenia virus (FPV) is a highly contagious viral disease of domestic cats and exotic cats. The virus is transmittable to susceptible cats by contact with body secretions and excretions of infected cats during the acute phase of infection in which virus is shed. Aerosol transmission, and transmission by insect vectors can also occur. The virus infects and destroys actively replicating cells in lymph nodes, and hematopoietic and gastrointestinal tissues of felines thereby causing sudden onset of symptoms including fever, anorexia, leukopenia, vomiting and diarrhea. In particular, FPV causes severe clinical illness in young kittens with high morbidity and mortality. A marked drop in total leukocyte count by day 4 to 6 after infection is the prominent indicator of FPV infection. Disease caused by infection with FPV has been described as feline parvovirus, feline panleukopenia, infectious enteritis, viral enteritis, cat "distemper", granulocytosis, cat plague, and cat fever.
FPV is a small, single stranded DNA virus, a parvovirus, that requires rapidly multiplying cells for DNA replication. The genome of FPV is a linear, single stranded DNA of about 5 kilobases in size that encodes three structural proteins: a large 80-85 kilodalton (kd) protein ("VP1") comprising 10% to 15% of the viral protein; a medium size protein of 64-67 kd ("VP2"); and a part of the VP2 protein which is converted to a 60-64 kd protein ("VP3") by proteolytic cleavage. The three proteins physically form a nested set of proteins within which the viral DNA is enclosed. FPV is very closely related to canine parvovirus (CPV) and mink enteritis virus both on the protein and amino acid level (Tratschin et al., 1982, J. Gen. Virol., 61:33-41; Truyen et al., 1994, Virology 200:494-503; and Truyen et al., 1994, J. Virol. 66:5399-5408). The high cross-reactivity between CPV and FPV indicated their antigenic similarities and the possibility of mutual neutralization and protection between the two viruses.
2. Rabies Virus Biology and Pathology
Rabies virus is a member of the genus Lyssavirus in the family Rhabdoviridae, and contains an unsegmented negative stranded RNA genome. Of the five known viral structural proteins, the rabies virus transmembrane glycoprotein G plays a critical role for the induction and binding of the virus-neutralization antibodies and the stimulation of T cell-mediated immunity (Lafon et al., 1983, J. Gen. Virol. 64:843-851; Lafon et al., 1985, J. Gen. Virol. 66:2125-2133; Wiktor et al., 1973, J. Immunol. 110:269-276; Wiktor et al., 1984, Dev. Biol. Stand. 57:199-211; Wiktor et al., 1984, Proc. Natl. Acad. Sci. U.S.A. 81:7194-7198). Also, the arginine, at position 333 in the glycoprotein amino acid sequence, is essential for the integrity of at least one antigenic determinant and for the ability of rabies virus to produce a lethal infection in adult mice (Dietzschold et al., 1983, Proc. Natl. Acad. Sci. U.S.A. 80:70-74). Initial symptoms of rabies virus infection include fever, and malaise. The disease progresses rapidly to symptoms including agitation, convulsions, and coma; and eventually, if untreated, the infected animal may die from organ failure.
3. Recombinant Veterinary Vaccines
In the art of veterinary vaccines, purified recombinant VP2 protein has been used as an immunogen for protecting dogs against infection by canine parvovirus (Wood et al., U.S. Pat. No. 4,971,793). In terms of viral vaccine vectors, vaccinia virus recombinants have been constructed with insertion of a respective gene encoding either feline infectious peritonitis virus (FIPV) spike protein, membrane glycoprotein, or nucleocapsid protein (Vennema et al., 1990, J. Virol. 64:1407-1409; Vennema et al., 1991, Virology 181:327-335). Immunization with such a vaccinia virus recombinant appeared to be of little or no value in the protection of vaccinated kittens against challenge with FIPV. Thus, vaccinia virus vectors do not appear to be a good choice for constructing feline recombinant vaccines because: (a) vaccinia virus/viral vectors did not elicit protection or detectable virus neutralization antibodies (Vennema et al., 1991, supra; Scott, 1988, Conf. Res. Workers Anim. Dis. 69:60); and (b) of concerns of introducing recombinant vaccinia virus for veterinary or human use, particularly because of rare side effects associated with vaccinia virus immunization.
In contrast, a raccoon poxvirus (RCNV) recombinant vector containing the gene encoding rabies virus surface glycoprotein G, has been used successfully to induce immunity in raccoons which is protective against subsequent challenge with raccoon rabies virus (Esposito et al., 1988, Virology, 165:313-316). However, the investigators report that the complete host range of RCNV is not known. A recombinant RCNV containing the gene encoding FPV VP2 protein ("recombinant RCNV/FPV") was recently constructed by inserting the VP2 protein gene into a vaccinia expression vector, and then recombining the insertion into the thymidine kinase (TK) gene of RCNV. In a vaccine trial, all cats immunized with the RCNV/FPV were fully protected against subsequent FPV challenge, and showed high titers of FPV viral neutralization antibody (U.S. patent application Ser. No. 08/190,789 assigned to the assignee of the present invention). Presently, there are no reports of the design of a functional multivalent RCNV-vectored vaccine, and its use for inducing protective immunity in felines.